1. Field of the Invention
The invention relates to a process for the preparation of pyridine-2,3-dicarboxylic acid esters using novel maleic acid esters as the starting materials. The pyridine-2,3-dicarboxylic acid esters, which may optionally be substituted in the 4-, 5- or 6-position, are themselves valuable intermediates for syntheses of herbicidally-active 2-(2-imidazolin-2-yl)-pyridine-3-carboxylic acid derivatives, as described in EP-B-41,623. Also the present invention relates to novel 2-amino-3-halomaleic acid esters, which are useful for producing the final product pyridine-2,3-dicarboxylic acid esters.
2. The Prior Art
The following processes are known for the preparation of pyridine-2,3-dicarboxylic acid esters:
1. Reaction of .alpha.,.beta.-unsaturated N,N-dialkylhydrazones with halomaleic acid derivatives to form 1-amino-1,4-dihydropyridines-2,3, dicarboxylic acid derivatives and subsequent treatment (thermal or acid treatment), the desired pyridine derivatives being produced with elimination of amine, as described in EP-A-161,221.
2. Reaction of .alpha.,.beta.-unsaturated aldehydes or ketones with .alpha.-halo-.beta.keto esters in the presence of ammonium salts, as described in DE-A-3,634,975, and
3. Reaction of .beta.-alkynyl-ketones with aminomaleates or aminofumarates to form pyridine-2,3-dicarboxylic acid esters which are substituted exclusively in the 6-position; 5,6-dimethylpyridine-2,3-dicarboxylic acid esters can in contrast be obtained by reacting 4-dimethyl-amino-3-methyl-butenone with amino fumarate, as described in GB-A-2,174,395.
Process (1) has the disadvantage that carcinogenic hydrazine derivatives are employed and the synthesis extends over several steps. In process (2), halooxalacetic esters which are not readily available are employed. These can be prepared by Claisen condensation of haloacetic esters and oxalic acid esters in moderate yields. Similarly, process (3) requires alkinyl ketones or alkenyl ketones containing terminal dimethylamino groups, which are likewise not readily accessible.